1. Field of the Invention
The present invention is directed to a ceramic-metal feedthrough lead assembly, and to a method for manufacturing such an assembly, and in particular to an assembly and method suitable for nerve or heart pacemakers.
2. Description of the Prior Art
Nerve or heart pacemakers are composed of essentially four components: an electronic circuit for generating the stimulation pulses and for processing control signals, a battery as the energy source, a hermetically tight encapsulation which protects the circuit and the energy source against body electrolytes, and electrodes consisting of a fatigue-resistant lead electrically insulated from the body, and terminating in an electrode head which produces the connection between the stimulation conductor (or conductors) within the lead and body tissue.
In the manufacture of such devices suitable for implantation in a patient, care must be exercised particularly in the manufacture of the ceramic feedthrough lead assembly for connecting the electrodes to the encapsulation or housing. Ceramic materials must be used which are resistant to body electrolytes, and which achieve a vacuum-tight, and thus moisture impermeable, connection. A potential loss of tightness is a particular disadvantage in such feedthrough lead assemblies, such a loss of tightness occurring as a result of crack formation in the ceramic which occurs in the vacuum-tight soldering of the metal and ceramic parts.
All previously known designs of ceramic-metal feedthrough lead assemblies are based on the assumption that the joined pieces must be very closely matched to each other in design and in their coefficients of expansion in order to resist crack formation during soldering. For this reason, conventional feedthrough lead assemblies exhibit very small gaps at the solder locations, these gaps being relatively easy to fill with solder. When soldering a titanium flange and a ceramic pearl with gold, however, titanium diffuses into the gold and vice versa. The resulting solder connection is thus extremely brittle and hard. Because the expansion of the titanium at the solder melting point is somewhat greater than that of the ceramic, the titanium contracts more than the ceramic during cooling. As a consequence, a pulling force is exerted on the ceramic, which leads to the formation of cracks in the ceramic because its tensile strength is relatively low. An interception of these tensile forces, however, is no longer possible at what is now a brittle solder connection.